5.2" Alethopteris Fern Plant Leaf Fossil Carboniferous Age Llewellyn FM ST Clair, PA

Location: Llewellyn Formation, St Clair, Pennsylvania

Weight: 9.6 Ounces

Dimensions: 5.2 Inches Long, 3.3 Inches Wide, 0.4 Inches Thick (Plate)

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Pennsylvanian Age, 300 million years old.

Alethopteris Sp Fern Fossil

The Llewellyn Formation in Pennsylvania, dating to the Pennsylvanian Period (~300 million years ago), is renowned for its exceptionally preserved fossil plants, particularly ferns. These fossils often appear in white or pale yellow against the dramatic backdrop of black shale, creating a striking contrast that highlights intricate leaf details.

These ancient plants fell into swampy environments, where low oxygen, high pressure, and low temperatures allowed their tissues to slowly mineralize. Initially, pyrite replaced the organic material, and over time, as sediments accumulated and heat and pressure increased, pyrophyllite (a whitish aluminum silicate mineral) replaced the pyrite. This process preserved the fine structural details of the ferns, making them some of the most visually impressive fossils from the Carboniferous period.

Commonly found genera include Alethopteris, Neuropteris, Pecopteris, and Sphenophyllum, many from plant families that are now extinct, such as Macroneuropteris. These fossils capture the diversity of Carboniferous swamp ecosystems, where ferns, horsetails, and club mosses dominated. By the end of the Carboniferous, tree-like club mosses and horsetails declined due to cooling climates and disappearing swamps, though coal swamps persisted into the early Permian, forming coal seams intertwined with fossilized plant layers.

Fossils are typically collected from thinly laminated shale, which splits easily with a chisel or hammer, making them accessible to both collectors and researchers. Llewellyn Formation ferns are prized for their scientific value, aesthetic appeal, and insight into prehistoric plant life.